This invention relates generally to digitally controlled impedance, and more particularly, to digitally switched impedance having enhanced linearity and faster settling times.
Digital potentiometers, sometimes referred to as xe2x80x9cvoltage-scaling digital-to-analog converters (xe2x80x9cDACsxe2x80x9d), are replacing analog potentiometers because they are smaller, more easily and accurately set, are controllable remotely, and are becoming lower in cost. The fineness of adjustment or xe2x80x9cgranularityxe2x80x9d of the digital potentiometer is determined by the number of xe2x80x9cdigital bitsxe2x80x9d used for the selection of the desired impedance value, i.e., 8 bits allows 256 different impedance selections, 10 bits allows 1024, etc. A disadvantage to finer adjustment granularity (more digital bits) is the rapid increase in the number of components (resistors, switches, decoders and logic circuits) required to implement the digital potentiometer.
Voltage-scaling DACs produce an analog output voltage by selectively tapping a voltage-divider resistor string connected between high and low reference voltages, with the low reference generally being set at ground. These types of converters are used most commonly as building blocks in metal oxide semiconductor (xe2x80x9cMOSxe2x80x9d) analog-to-digital conversion systems, where they function as the DAC subsection of a successive-approximation-type analog-to-digital converter. For an N-bit voltage-scaling DAC, the resistor string consists of 2N identical resistors connected in series, and the DAC is used as a potentiometer in which the voltage levels between the successive series-connected resistors are sampled by means of binary switches. Replacing mechanical potentiometers and rheostats is an important and potentially very high volume application for these devices.
FIG. 1 is a schematic diagram of an N-bit DAC that operates on the voltage-scaling principle. A resistor string consisting of series-connected resistors R1, R2, R3, . . . , R2Nxe2x88x921, R2N is connected between a high reference voltage (VREF+) node 2 and a low reference voltage (VREFxe2x88x92) node 4, which are typically 5 volts and ground potential, respectively. The voltage drop across each resistor is equal to one least significant bit (LSB) of output voltage change. The output is sampled by a decoding switch network, illustrated as switches S1, S2, S3, . . . , S2N. Each switch taps a different point in the resistor string, so that closing a particular switch while leaving the other switches open places a unique analog voltage on a common output line 6 to which each of the switches is connected. A decoder (not shown) controls the operation of the switches so that the switch whose voltage corresponds to the magnitude of the input digital signal is closed. The signal on analog output line 6 may be sensed by a high-impedance buffer amplifier or voltage follower A1, the output of which is connected to an output terminal 8 that provides the final output analog voltage. To ensure the accuracy of the conversion, the buffer amplifier should draw negligible DC bias current compared to the current within the resistor string. A principal drawback of this type of circuit for high-bit-count D/A conversions is the very large number of components required: 2N resistors, 2N switches and 2N logic drive lines. For example, in a 12-bit implementation, this approach would use 4,096 resistors, 4,096 switches and 4,096 logic drive lines. It is highly desirable to significantly reduce this large number of elements for purposes of area savings, higher manufacturing yields and lower costs.
Voltage-scaling DACs are presently available which greatly reduce the number of required resistors and switches by using one resistor string consisting of 2N/2 resistors for the input digital signal""s most significant bits (MSBs), and a separate resistor string also consisting of 2N/2 resistors for the least significant bits (LSBs). Each resistor in the LSB string has a impedance value equal to xc2xdN/2 the impedance of each MSB resistor. The opposite ends of the LSB string are connected across one of the MSB resistors. By varying the MSB resistor selected for the LSB string connection and taking an output from the LSB string, outputs in one LSB increments can be obtained over the full range of one to 2N/2xe2x88x921 LSBs.
A reduced parts count resistor-switch configuration for a digital potentiometer is disclosed in U.S. Pat. No. 5,495,245 by James J. Ashe. Referring now to FIG. 2, the digital potentiometer disclosed in the Ashe patent uses two outer strings 10 and 12 to provide a decremented voltage pattern that supplies an analog signal corresponding to the MSBs of the input digital signal while an inner string 14 provides an analog signal corresponding to the LSBs; alternately, the outer strings can provide the LSBs and the inner string the MSBs. The two outer strings 10 and 12 are identical, with the high voltage end of the first outer string connected to the high reference voltage, VREF+ and the low voltage end of the second outer string 12 connected to the low reference voltage, VREFxe2x88x92. The opposite ends of the inner string 14 are connected to the first and second outer strings, 10 and 12, through respective outer switch networks that are operated by a decoder (not illustrated), the decoder in effect causes the opposite ends of the inner string to xe2x80x9cslidexe2x80x9d along the two outer strings. This xe2x80x9cslidingxe2x80x9d keeps a constant number of outer string resistors in the circuit, regardless of where the outer strings are tapped. No active elements are required to buffer the inner string from the outer string, which allows the circuit disclosed to be used as a potentiometer or rheostat. The output voltage is obtained by tapping a desired location in the inner string 14. In the Ashe invention, regardless of whether the MSB values are produced by the inner or outer strings, each MSB resistor string includes 2N/2xe2x88x921 resistors of impedance value R, and 2N/2 switches. Each LSB string includes 2N/2 resistors of impedance value R/2N/2, and 2N/2 switches. The Ashe digital potentiometer results in a significant reduction in the number of both resistors and switches, compared to the potentiometer circuit illustrated in FIG. 1.
The digital potentiometer disclosed in Ashe has inherent non-linearity due to resistor, interconnect and switch impedance mismatches, and also long switching settling times caused by large internal capacitance from the parallel connected switches located on the output taps of the MSB resistor strings.
Accurate and linear adjustment of impedance is also highly desired and has many applications for adjustment of tuned circuits, frequency tuning, impedance matching, radio frequency and audio applications, alternating current circuit applications, signal phase shift adjustment, frequency response compensation and the like. Impedance may be comprised of resistance and/or reactance. Reactance may be either capacitive or inductive. Selective combinations of resistance, capacitance and inductance enable a wide range of impedance values. Impedance is generally designated by the letter xe2x80x9cZxe2x80x9d and shall be used herein as any combination of resistance and/or reactance (either capacitive or inductive). Impedance is measured in Ohms and may be expressed in either polar (Z∠"PHgr") or rectangular (R+jX) format.
Therefore, what is needed is a digitally switched impedance which retains the simplicity and economy of having a reduced number of impedances and switches in a combination of major and minor impedance strings and switches, but having improved linearity and reduced settling times when impedance values are switched.
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing a digitally switched impedance having improved linearity and reduced settling times when the impedance values are switched. An embodiment of the present invention digitally switched impedance may be fabricated on an integrated circuit die using complementary metal oxide semiconductor (CMOS) transistors for the switches.
An embodiment of the invention uses two scaled minor impedance strings (LSB) as the upper and lower ranks, and a major impedance string (MSB) as the bridge rank connected between the upper and lower ranks. The switches for the upper and lower ranks are connected between the respective voltage references and the series-connected impedances of the upper and lower ranks. Additional switches are connected from the bridge rank (MSB) impedances to the output node (wiper) of the digital potentiometer. The MSB portion of the digital value is selected with one of the bridge rank switches, and the LSB portion of the digital value is selected with a pair of switches connected to the upper and lower ranks. A varying portion of the upper and lower ranks are connected with the bridge rank, and the total available tap points equals the product of the number of taps on the bridge rank times the number of taps on one of the other (upper or lower) ranks
The overall linearity of the digitally switched impedance circuit of the present invention is significantly improved over the prior art digital potentiometer circuits because the majority of the total impedance is always used for the total impedance value of the digitally switched impedance circuit. Unlike the prior art digital potentiometers where up to 50 percent of the total resistance involves the swapping of resistances. The matching requirements of the upper and lower ranks are now reduced to the scaled impedance values, and the matching level to guarantee monotonicity is also reduced by the same factor.
Also, all the switches in the upper rank see the same constant biasing voltage (meaning they have the same constant impedance) as opposed to what happens in the prior art where the biasing voltage of each switch varies with its position in the rank. The same also is true for the switches in the lower rank. Thus, there is no need to size each switch independently to match all the switches impedances.
Alternating current (AC) performance of the present invention is also improved over the prior art because the voltage levels at the upper and lower rank switches are now limited to a small fraction of their former range, that fraction being one over the number of impedances in the major rank (bridge rank). A reduction in the capacitance contribution from the switches results in better settling time and improved AC response. The switch placement of the present invention further improves the AC performance by removing the switch capacitance from the settling nodes of the common signal bus. The settling time is now only affected by the capacitance of the impedances of all ranks and just the bridge rank switches.
Another embodiment of the invention uses two scaled major impedance strings (MSB) as the upper and lower ranks, and a minor impedance string (LSB) as the bridge rank connected between the upper and lower ranks. The switches for the upper and lower ranks are connected between the respective voltage references and the series-connected impedances of the upper and lower ranks. Additional switches are connected from the bridge rank impedances to the output node (wiper) of the digitally switched impedance. The LSB portion of the digital value is selected with one of the bridge rank switches, and the MSB portion of the digital value is selected with a pair of switches connected to the upper and lower ranks.
Features and advantages of the invention will be apparent from the following description of presently preferred embodiments, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.